If any avatar (alt or not) is abusive to you in IMs or griefing you, file an Abuse Report (AR) with Linden Lab. If they show up at your fishing contest and are unwelcome, kick and ban them from your land. If they abuse you in Social Chat, our group chat room, file an AR and also send a complete chat log to 7Seas Sass for our consideration.
The latter is the only thing that we have jurisdiction over; we can't start policing whose alt is whose or punishing behavior that happens outside of our chatroom or on other people's land. Thanks for your understanding.